Conventionally, there has been developed a plasma generation device that generates local plasma by way of a spark plug discharge and enlarges the plasma by way of an electromagnetic wave such as a microwave (for example, see Japanese Unexamined Patent Application, Publication No. 2009-036198). The plasma generation device is provided with a mixing circuit that mixes a discharge current for a spark discharge (energy for the discharge) and energy of the electromagnetic wave from an electromagnetic wave generation device. The mixing circuit is electrically connected with a connection terminal part serving as an input terminal of the spark plug. As a result of this, a high voltage pulse (the discharge current) for the spark discharge and the electromagnetic wave are supplied to the spark plug through a same transmission line (electric path). Accordingly, the central electrode of the spark plug serves as both a spark discharge electrode and an antenna for electromagnetic wave emission.
However, a central electrode of a spark plug (hereinafter, in the spark plug, a whole portion extending from a terminal part connected with an ignition coil up to a tip end part that forms a discharge gap with a ground electrode is referred to as the “central electrode”) generally used in a conventional plasma generation device is usually constituted by an iron-based alloy except in the tip end part. The electromagnetic wave provided from an alternating current power supply flows on a surface of the central electrode, the principal component of which is iron having a high magnetic permeability, resulting in a great power loss. Therefore, it has been difficult to downsize an electromagnetic wave oscillator.
Furthermore, the discharge current for the spark discharge and the electromagnetic wave are both emitted from the tip end part of the central electrode. Accordingly, between the tip end of the central electrode and the ground electrode, the electric fields caused by the discharge current and the electromagnetic wave culminate in intensity at an axial center part of the central electrode.
More particularly, the intensity of the electric field between the tip end of the central electrode and the ground electrode caused by the discharge current and the electromagnetic wave distributes in such a curved manner as to be symmetric about and culminating at the axial center of the central electrode and declining toward outer peripheries of an insulator that covers the central electrode as shown in FIG. 5. Accordingly, the electric field caused by the discharge current is superimposed on the electric field caused by the electromagnetic wave, thereby further increasing the electric field intensity, and the temperature becomes maximum at the axial center of the central electrode. As a result of this, there has been a problem such that the tip end part of the central electrode is prone to erosion.